SAM error message re # modules per string

I'm using the following system in CEC Performance Model with Module Database:

SunPower SPR-X22-370, Heat-transfer method, rack mount, module dimensions
SMA America SB6000TL-US (240V) inverter
No shading and no input losses.

In system design, when I enter 6 modules per string and 2 strings in subarray 1, nothing in other subarrays, it works fine and reports 9,102 kWh/yr.

But this is not the real design. When I try to enter it correctly as 12 modules per string and 1 string, it gives the following error message:

Module array voltage Vmp exceeds the Vdcmax (480.00V) of inverter 4215 times.
The maximum Vmp value is 743.03V at hour 968.
It is recommended that you reduce the number of modules per string.
Inverter oversized: The maximum inverter output was 55.39% of the rated value 6 kWac.

I've tried downsizing the inverter, but although it removes the last line of the error message, the rest of the errors persist.

Please explain and tell me how to do this correctly.

Dear Dennis,

That message is a "notice" rather than a "warning" or "error," so you should consider it to be informational and ignore it if the results are acceptable.

For your design of 12 modules per setring, the nominal string voltage at maximum power is 12 modules * 59.6 VDC = 715.2 VDC. The inverter's maximum MPPT voltage rating is 480 VDC, so your 12-module design results in nominal maximum power voltage that exceeds the inverter rating. The message under "Electrical sizing information" on the System Design input page tells you that.

SAM does not prevent you from running a simulation for that design, and when you do, provides additional information in the notice. In this case, the maximum operating voltage of the array was 743.03 VDC in the 968th hour of the year. If you look on the Losses tab of the results page, you should also see a large power loss due to inverter MPPT clipping.

If you want to avoid that power loss, you could reduce the number of modules per string for example, 6 modules per string and 4 strings in parallel maintains the same power rating and eliminates the power loss from clipping. Or, you could choose an inverter with a maximum voltage rating above the array's nominal Vmp = 715 VDC like the Ingecon 6 TL.

You can also run the System Sizing macro to generate a more detailed report to help you choose a combination of module and inverter that meets your requirements. To run the macro, click Macros (under the Simulate button), and then click System Sizing.

Best regards,
Paul.